Fire-resistant, multiple pane glazings, formed of at least two spaced apart parallel glass sheets having an aqueous thermal barrier therebetween, were originally developed for forming port holes or the walls of glove boxes used in the nuclear industry. Since water, which has a high heat of vaporization, is essentially transparent, it was initially utilized as a thermal barrier between the glass sheets of such glazings. Another advantage to the use of water is that it has an excellent neutron section which, in the event of a shock or fire, would be of assistance in preventing the contamination of the environment external to the glazing with dangerous radioactivity. A disadvantage to the use of water for such purposes, however, is that in case of accident, shock or fire the glass sheets may become broken, thus permitting the water to flow away from where it is most needed.
Fire proof glazings have also been constructed by placing a solid layer of a material between the two sheets of glass which, in response to heat radiation, is transformed into an insulating foam. One example of such a composition is a hydrated alkaline silicate. Such glazings are described in French Pat. No. 2,027,646. Their optical properties and, in particular, their transparency, leave something to be desired however. Further, they do not have the capability to remain fireproof for thirty minutes except when certain methods of construction are used in which, for example, the layer of silicate is reinforced with glass fibres or several separate panes are mounted in the same frame in order to form a multiple pane glazing, which reduces the optical quality even further.
In order to eliminate the risk of water leakage, glazings have been constructed with the space between the glass sheets filled with an aqueous gel. These gels are formed by a polymer which is present in the form of a network of microcavities which may be open or closed and which contain a liquid, preferably water. French Pat. No. 1,458,945 discloses the use of such transparent polymer gels. The network of closed microcavities prevents the gel and the liquid it contains from flowing away if the glass sheets should break. However, for the thicknesses compatible with weight and cost requirements utilized in the construction industry, the fire resistance of these glazings is not sufficient to fulfill the requirements of fire security standards which are required by many municipal authorities.
One such commonly required fire security standard, well-known to those of ordinary skill in this area, is DIN Standard 4102. This standard defines a method of testing and the criteria to which glass window panes for buildings must conform in order to be classified as "fire resistant". Materials tested according to this standard are classified by the time during which, under standard trial conditions, they remain capable of performing the following functions:
sufficient mechanical strength for the element considered to continue to carry out its function, PA1 thermal insulation, PA1 resistance to flames, PA1 absence of the emission of inflammable gas from the surface exposed to heat during the trial
The thermal insulation property of a material is considered satisfactory when the mean heating of the nonexposed face, i.e., the mean of the temperatures recorded on the non-exposed face and the maximum heating, i.e., the maximum temperature indicated by the least favorable thermo-couple arranged on the non-exposed face do not exceed, respectively, 140.degree. and 180.degree. C. The "fireproof" elements are those for which all the criteria mentioned above are met.
In each category the classification expresses, as a function of the time during which the elements satisfy the trials, the degree of retention of the above-mentioned properties. The "degree" is the average time equal to or immediately less than the duration during which the element has met the properties required. For applications in the building construction industry the requirement is for windowpanes having "fireproof" characteristics for at least thirty minutes without the need to make the window excessively thick.
A further improvement to the use of polymerized aqueous gels for this purpose was disclosed in British Pat. No. 1,541,371 published Feb. 28, 1979, which is an English language equivalent of German Patent Application No. 2,713,849 cited in the priority application. The thermal barrier disclosed therein comprises a hydrogel composed of about 70-90% by weight of water and about 10-30% by weight of a water soluble salt. The gel may advantageously be based on a derivative of acrylic acid, and the water soluble salt may be chosen from the group formed by aluminates, silicates, stannates, plumbates, alums, borates, phosphates and other salts of an alkali metal or ammonium.
The flame retardant effect of such glazings lies in the fact that initially, considerable amounts of thermal energy are absorbed by evaporation of the water. During the period in which the evaporation takes place, the temperature of the glazing is raised only slightly on the surface which is removed from direct contact with the heat. It remains substantially below the acceptable DIN 4012 value, which is about 140.degree. C. below the initial temperature.
Once the water has evaporated, a foamy protective shield is formed by the residual salt compound. This shield thus prevents the penetration of thermal radiation through the glazing. By varying the thickness of the gel layer, glazings may be produced having a fire resistance corresponding to F30 or F60 according to the requirements of DIN 4102, Part II. Thus, when a glazing constructed in this manner is subjected to a flame, it has been found that the glass sheet which is exposed to the fire breaks rapidly. Subsequently, however, the gel layer forms a screen which prevents the heat from spreading and reaching the second pane. Glazings of this type which are constructed with three glass plates may even attain a fire resistant classification of F90.
To form an effective shield against the thermal currents normally produced by a fire, a sufficiently large amount of the salt must be contained in the gel layer for a coherent foam of sufficient consistency to be formed. Consequently, the salt must have a proportionally high solubility in water. In addition, the salt solution, as well as the structure of the polymer forming the gel, should not exhibit cloudiness or coloring, i.e., it should be markedly transparent. The salts particularly suitable for practical use under these conditions, namely, chlorides of sodium, calcium, magnesium or other salts, exert a strongly aggressive and corrosive effect on the metal of the frame separating the glass plates. Even with the use of various steels or other metals such as nickel and/or chromium, which are corrosion resistant, corrosion can appear on the separating frame and, if unfavorable conditions are encountered, the products of the corrosion can be dissolved in the gel leading to localized discoloration and/or cloudiness of the gel layer.
Applicant's have solved the problem described above by developing a fire-resistant glazing which prevents the formation of discoloration or cloudiness in the gel caused by corrosion of the metal separation frame. To eliminate such disadvantageous clouding of the glazing, a water soluble anticorrosive composition is advantageously added to the hydrogel.